Form ŝīrɛ̄ seems a misprint for šīrɛ̄ judging by the form šīrέh quoted in the context given in [ML 395] and the fact that the word is placed under šrˁ.
Note that -ˁ, absent in other Sem., may be due to contamination either with méŝeräˁ ‘tendon of Achilles’ (SSL LS 1473) or with such forms attested only in Jib.